The present invention relates to free piston engines.
Conventionally, internal combustion engines have operated with the motion of the pistons mechanically fixed. For example, a conventional internal combustion engine for a motor vehicle includes a crankshaft and connecting rod assemblies that mechanically determine the motion of each piston within its respective cylinder. This type of engine is desirable because the position of each piston is know for any given point in the engine cycle, which simplifies timing and operation of the engine. While these conventional types of engines have seen great improvements in efficiency in recent years, due to the nature of the engines, that efficiency is still limited. In particular, the power density is limited because the mechanically fixed motion of the pistons fixes the compression ratio. Moreover, all of the moving parts that direct the movement of the pistons (and camshafts and engine valves as well) create a great deal of friction, which takes energy from the engine itself to overcome. The resulting lower power density means that the engine will be larger and heavier than is desired. Also, the flexibility in the engine design and packaging is limited because of all of the mechanical connections that must be made.
Consequently, is desirable, for environmental and other reasons, to have an engine with a higher power density than these conventional engines. The advantages of lighter relative weight, smaller package size, and improved fuel efficiency can be a great advantage in both vehicle and stationary power production applications.
Another type of internal combustion engine is a free piston engine. This is an engine where the movement of the pistons in the cylinders is not mechanically fixed. The movement is controlled by the balance of forces acting on each piston at any given time. Since the motion is not fixed, the engines can have variable compression ratios, which allow for more flexibility in designing the engine's operating parameters. Also, since there are no conventional crankshafts and rods attached to the crankshaft, which reduces piston side force, there is generally less friction produced during engine operation. However, since the piston motion is not mechanically fixed, the piston positions and velocities must be monitored in order to allow the control system to start and maintain engine operation. Without accurate information on the piston position and velocity, the timing for fuel injection cannot be determined.
Moreover, an opposed piston, opposed cylinder (OPOC) configuration of a free piston engine is particularly desirable due to its inherently balanced operation—with a compact layout as well. Here, there are two sets of piston assemblies that must move appropriately relative to the cylinders and to each other. Without accurate information relating to the position and velocity of the piston assemblies, the engine will not operate. However, it is not desirable to add a great deal of complexity and cost to the engine. In addition, the sensors need to be able to operate over long periods, while retaining the accuracy needed for precise engine operation, even in the harsh engine environment where high temperatures, lubrication oil, and other conditions may hamper accurate position and velocity readings. And, this accurate sensing needs to be accomplished while measuring the piston assembly positions over the entire length of a relatively long piston stroke.
Thus, in order to obtain the efficiency benefits of a free piston engine and in particular an OPOC free piston engine, it is desirable to have a reliable, accurate and relatively simple way to determine the position and velocities of the pistons as they reciprocate in the engine through their entire piston strokes.